Pulse frequency modulation (PFM) is a switching method commonly used in many DC-DC voltage converters to improve efficiency at light loads. PFM is often used at light load currents and pulse width modulation (PWM) is used at heavier load currents. This type of operation allows the converter to maintain high efficiency over a wide range of output current. In PFM mode, the converter begins switching only when the output voltage falls below a lower threshold voltage value and continues to do so until it reaches an upper threshold value. Once the output reaches the higher threshold value, all unnecessary internal circuitry is turned off to reduce the quiescent current. This control method significantly reduces the quiescent current to a typical value of 20 μA, which results in a higher efficiency at light loads.
In PFM mode, a series of inductor current pulses are applied to the load and output capacitor to maintain the output voltage within preset boundaries. This mode effectively lowers the frequency of the switching-cycle events, thereby lowering the switching losses in the converter. There are several variations on PFM, such as single-pulse PFM, multipulse PFM and burst-mode PFM. However, all operate according to the basic principle of initiating switching cycles only as needed to maintain the output voltage. However, there are heretofore unaddressed needs with previous solutions in managing the spectral content of the output of the converter.